Principles of Potash Use

Principles of manuring

It is the soil that feeds the crop, while fertilisers feed the soil.

Critical values of soil K for different crops have been established from numerous experiments as illustrated in Figures 6 and 7 at which full crop yield will be achieved. For all soils, except the lightest textured soils which cannot sustain these levels without risk of leaching, the aim is to maintain soil K above the critical level appropriate to the rotation, using fertiliser and manure to replace the potassium removed by cropping.

It should be recognised, therefore, that potash is unlike most other farm inputs, which are justified on the basis of direct yield response and financial return per £ spent. Correct use of potash to replace crop offtake and maintain soil reserves may not result in an immediate yield response. Failure to replace the potassium removed in the crop and maintain an adequate soil reserve prejudices future yields and crop quality and unnecessarily jeopardises the response to all other inputs.

Policy for the lightest textured soils (sands and loamy sands) should be to ensure at least the replacement of removals but not to expect to be able to raise the inherently low levels of soil K to the same values as for other soils.

Benefits of maintaining soil fertility

• Fertile soils have a better distribution of nutrients throughout the soil profile, and this cannot be achieved immediately from fresh fertiliser applications. This results in more effective uptake, especially for poorer rooting crops such as potatoes, and better yields than can be achieved from low soil fertility and high fertiliser regimes.
• Soils with adequate nutrient levels avoid the risk of damage to roots from local, high concentrations of salts from large fertiliser applications.
• Satisfactory fertility removes the urgency of timeliness in application of fertilisers, which eases peak work-loads, offers cash flow benefits and allows fertiliser application to be undertaken at the best time without risk to soil structure or seedbeds from tractor wheelings. Crop response to nitrogen fertiliser depends on having appropriate amounts of other nutrients (e.g. P, K, S, and Mg) in the soil profile explored by roots. Shortages of these nutrients will result in poorer responses to nitrogen and the risk of leaving unused nitrogen residues in soil.

Target levels of soil fertility

* Soil target concentrations are for Olsen P. Some laboratories use other extractants.
** These soils cannot be maintained at higher levels of K without risking unacceptable losses. Regular use of organic manure is of considerable benefit in these situations.

Soil K levels cannot be rapidly changed up and down to suit individual crops and the appropriate target level must be set for the most demanding crop in the rotation.